The present invention relates generally to disc grinders for lignocellulosic material. More particularly, the present invention relates to refiner plate segments for such an apparatus.
In high consistency mechanical pulp refiners, the wood fibers are worked between two relatively rotating discs on which refiner plates are mounted. The plates usually have radial bars and grooves. A large volume of steam is produced between the plates as a result of this refining work. For effective refining, the fibers must be retained between the plates on the bar surfaces despite the high velocity of the flowing steam, and the enormous centrifugal forces. Typically, dams are provided in the grooves to interrupt material flow and thus improve the retention time of the material in the refining region.
In a typical refiner plate with radial bars and grooves, the bars provide impacts or pressure pulses which separate and fibrillate the fibers. The grooves enable feeding of the fibers and steam extraction. Near the perimeter of the plates, high radial steam flow and high centrifugal force both act to sweep the fibers outwardly from between the plates prematurely, thus reducing the refining effectiveness. The flow restrictions due to a small plate gap and fiber-filled grooves result in a steam pressure peak between the plates, located radially inward from the perimeter. This pressure peak is a major source of the refining thrust load, and can induce control instability at high motor loads. Dams in the grooves help to retain the fibers and force them to the bar surface. However, they also further restrict the steam flow.
It is thus desirable that the steam generated during refining be discharged from the refining region as quickly as possible, while retaining the pulp within the region as long as possible. Conventional refiner plates utilize a variety of mechanisms to promote the flow of steam while retarding the flow of pulp. U.S. Pat. No. 1,131,272 discloses refiner plates having dams, or baffles, that have a sloping upper surface. The slope of alternate baffles in the same groove is reversed, causing the steam/pulp mixture to take a zig-zag path. Such refiner plates are costly to manufacture. More recent efforts include refiner plates which utilize exhaust channels to assist in the removal of steam, as shown in U.S. Pat. No. 4,676,440. Another design is disclosed in U.S. Pat. No. 4,953,796 and U.S. Pat. No. 5,373,995, whereby apertures in or spaces between the bars enable steam in a partially blocked groove to flow into an adjacent groove.